Basically, my internet is shit, and my brothers is fast.

So in steam you're able to transfer data to the folder and it will verify and you don't need to download the game.

I was wondering if GOG has the same feature.

So my brother will complete the download, I'm going to transfer it to a external drive and transfer it through GOG and it will work is my question?

Gog allows you to download an offline package for the game. I'm not sure if it is possible to link it then to the galaxy client though.

Sorry, maybe what i said was a bit confusing.

I'll buy the game through GOG as well since my brother bought it here. But instead of downloading the files. I'll transfer it via USB drive from his computer to mine.

There is a verification feature, at least with 2.0. I've needed to use it several times while testing running games via a Galaxy client which was itself running via Lutris. Saves redownloading everything when things break.

From the game page in your library, click on the two-slider icon in the upper right, then Manage Installation, then Import folder. I've seen this option greyed out before though, so your mileage may vary.

As far as I know, GOG still uses their installer for everything except maybe GWENT. You should be able to install/update Cyberpunk with that, although I'm not sure how much Galaxy checks when it verifies new games it did not install itself. GOG Galaxy puts these installers in its download directory.

To get these installers you can also use the installer links in your library, or some of the 3rd party GOG installer downloader. I use lgogdownloader, which is for Linux only, but works great in a Virtualbox VM (never tried if it runs on WSL).